1. Field of the Invention
The present invention relates generally to continuously variable transmissions and more specifically to hydraulic control thereof.
2. Background Art
The invention is particularly, although not exclusively, applicable to transmissions which incorporate a ratio varying unit (“variator”) of the toroidal-race rolling traction type to provide the required continuously-variable transmission ratio. Major components of a known variator 10 of the “full toroidal” type are illustrated in FIG. 1. Here, two input discs 12, 14 are mounted upon a drive shaft 16 for rotation therewith and have respective part toroidal surfaces 18, 20 facing toward corresponding part toroidal surfaces 22, 24 formed upon a central output disc 26. The output disc is journalled such as to be rotatable independently of the shaft 16. Drive from an engine or other prime mover, input via the shaft 16 and input discs 12, 14, is transferred to the output disc 26 via a set of rollers disposed in the toroidal cavities. A single representative roller 29 is illustrated but typically three such rollers are provided in both cavities. An end load applied across the input discs 12, 14 by a hydraulic ram 15 provides contact forces between rollers and discs to enable such transfer of drive. Drive is taken from the output disc to further parts of the transmission, typically an epicyclic mixer, as is well known in the art and described e.g. in UK patent application 8429823. Each roller is journalled in a respective carriage 30 which is itself coupled to a hydraulic actuator 32 whereby an adjustable translational force can be applied to the roller/carriage combination. As well as being capable of translational motion the roller/carriage combination is able to rotate about an axis determined by the hydraulic actuator 32 to change the “tilt angle” of the roller and to move the contacts between rollers and discs, thereby varying the variator transmission ratio, as is well known to those skilled in the art.
The illustrated variator is of the type known in the art as “torque control”. The hydraulic actuator 32 exerts a controlled force on the roller/carriage and for equilibrium this must be balanced by the reaction force upon the roller resulting from the torques transmitted between the disc surfaces 18, 20, 22, 24 and the roller 28. As is well known in the art, the center of the roller is constrained to follow the center circle of the torus defined by the relevant pair of discs. The axis determined by the actuator 32 is angled to the plane of this center circle. This angle is referred to as the “castor angle”. The well known result of this arrangement is that in use each roller automatically moves and precesses to the location and tilt angle required to transmit a torque determined by the biasing force from the actuator 32.
The biasing force is controlled by means of a hydraulic circuit through which fluid is supplied to the actuators at variable pressure.
It will be appreciated that while the equilibrium position of the rollers is uniquely determined by the balance of the reaction force and the applied biasing force, there is the potential for unwanted oscillatory motion of the roller/carriage combination about this position, with resulting impairment of transmission function. More than one mode of oscillation is possible but in the simplest such mode all rollers oscillate in unison and this oscillatory motion is accompanied by a corresponding flow of fluid in the hydraulic circuit.
Damping of such oscillation can be provided by means of the hydraulic circuit and specifically by suitably restricting or throttling fluid flow to and from the actuators 32. Such restriction of flow can tend to restrict the motion of the rollers required to effect ratio change, but it has been found that in a lightly damped system these conflicting requirements can be satisfied in a manner which is entirely satisfactory under the majority of operating conditions.
However, particularly stringent requirements are imposed on the transmission during very rapid changes of vehicle speed, particularly in the case of an emergency rapid “brake to rest” e.g. an emergency stop. In order to maintain engine speed and to avoid stalling the engine, rapid ratio change is required of the variator. This is particularly significant in a transmission of the “geared neutral” type in which the variator remains coupled to the vehicles wheels even while the wheels are stationary—that is, in vehicles lacking a clutch or other means to isolate wheels and engine. The high rate of ratio change required during a rapid brake to rest corresponds to a rapid motion of the variator rollers, but if adequate hydraulic flow to accommodate such motion is not available—particularly because such flow is restricted—the rollers can fail to move with sufficient speed, leading e.g. to an engine stall. Within the hydraulic circuit the effect can be a large increase in pressure on one side of the circuit and a large fall in pressure on the other side of the circuit. The result must be a large net biasing force on the roller/carriage combinations and this is reflected in a large variator torque which is the cause of the engine stall.
The applicant's own European patent 1076786 and its US counterpart application Ser. No. 09/678,483 describe a hydraulic variator control circuit in which, by appropriate setting of certain valves, a connection can be made from one side of the actuator 32 to the other, allowing rapid movement of the actuator and its roller in order to effect rapid ratio change. However the switching valves used in this arrangement provide no control of the resulting flow—control of the variator is effectively lost when the valves are set as just mentioned. The valves used to normally control hydraulic pressures applied to the variator are rendered ineffective. This is incompatible with maintenance of stable variator control.